Hypereutectoid rail and manufacturing method thereof

ABSTRACT

The invention relates to the field of railway, and discloses a hypereutectoid rail and a manufacturing method thereof, which includes rolling a steel billet containing V and Ti, wherein, based on its total weight, the steel billet contains C of 0.85-0.94 wt %, and the relation between the start rolling temperature Tstart/final rolling temperature Tfinal and the content of V and the content of Ti satisfies the following formulas: Tstart=1100+a([V]+5[Ti]), Tfinal=750+b([V]+5[Ti]), wherein, 500≤a≤800, 300≤b≤500; based on its total weight, the steel billet contains V of 0.03-0.08 wt %, Ti of 0.011-0.02 wt %, and [V]+5[Ti]: 0.12-0.14 wt %. The hypereutectoid rail manufactured with the method has excellent comprehensive performance of strength and toughness.

FIELD OF THE INVENTION

The invention relates to the field of railway, and particularly a hypereutectoid rail and manufacturing method thereof.

BACKGROUND OF THE INVENTION

Pearlitic rail is the most widely used product in railway field. As the axle load of railway and the traffic density increase, the maximum strength of existing ordinary carbon rail and microalloyed rail with content of hypereutectoid carbon is only 1,300-1,400 MPa, even though heat treatment is conducted. Under the action of the multi-direction and complex heavy load between the wheel and the rail, the railhead is prone to have defects and failures such as rapid wear, nucleus flaw, peeling off, etc., which significantly reduces the service life of rail and endangers traffic safety. Therefore, it is very important to improve the obdurability of rail and prolong its service life when the harsh transportation conditions are irreversible. For small radius curve sections of heavy-haul railway, wear has gradually become the primary factor affecting the service life of railway. Research has proved that the most effective way to improve wear resistance is to improve the hardness of rail. For the existing pearlitic rail, the most effective and the cheapest way to improve hardness is to increase the carbon content in steel. However, increase of carbon content will lead to a reduction of the toughness and plasticity of rail, which already approaches the lower limit for safe service of rail. If the toughness and plasticity further degrade, risk for brittle fracture of rail will increase under the same service conditions. By adopting heat treatment technology and relying on the strong effect of fine-grain strengthening, it is possible to improve the toughness and plasticity of rail while improving the hardness. It is proved in practice that heat treated rail generally has a better comprehensive performance of strength and toughness than hot rolled rail with the same composition. Nevertheless, if the content of carbon further increases, for example to above 0.85%, the improvement of toughness and plasticity is limited even though heat treatment technology is adopted, and new technology should be applied to meet the requirements for the obdurability of rail containing hypereutectoid carbon.

SUMMARY OF THE INVENTION

The purpose of the invention is to overcome the problem of poor strength and toughness of the hypereutectoid rail manufactured with existing technology, and to provide a hypereutectoid rail and its manufacturing method.

During the research, the inventor of the invention found out that, for the high-carbon rail, refining austenite grains and finally refining the lamellar spacing of pearlite not only improved the strength of rail, but also significantly improved the toughness and plasticity. Although the improvement of toughness and plasticity through refining austenite grains is far less effective than that through fine-grain strengthening, for the hypereutectoid rail with higher carbon content, which has the toughness and plasticity approaching the limit, it is of great significance to obtain higher toughness and plasticity while maintaining high strength if the austenite grains can be further refined. For the refining of austenite grains, the following method is described in CN102803536A, Pearlite-based High-carbon Steel Rail Having Excellent Ductility And Process For Production Thereof: {circumflex over (1)} Reheating the rail at a low temperature after rolling. But it has a problem that melting takes place inside of austenite grains, leaving coarse carbides and degrading the toughness and plasticity of the pearlitic structure after accelerated cooling. Meanwhile, reheating also has economic problems such as high manufacturing cost and low production efficiency. {circumflex over (2)} The obdurability of rail product is improved through the precipitates pinning at the austenite grain boundaries and by refining the austenite grains. But researches indicate that the state of austenite has very limited influence on the performance of the final product for the rail containing hypereutectoid carbon. However, when the content of carbon exceeds 0.85%, the state of austenite has significant influence on the performance of product.

During the research, the inventor of the invention also found out that, for the purpose for refining austenite grains and obtaining rail products with excellent performance, the start rolling temperature and the final rolling temperature should be controlled in a unified and coordinated manner, so as to manufacture rails with excellent comprehensive performance of obdurability under hypereutectoid condition. If combined with controlling the chemical composition of the steel billet and the cooling process at the same time, rails with better performance will be manufactured.

Therefore, the invention provides a manufacturing method for hypereutectoid rail, wherein, the method comprises rolling a steel billet containing V and Ti, wherein, based on its total weight, the steel billet contains C of 0.85-0.94 wt %, and the relation between the start rolling temperature T_(start)/the final rolling temperature T_(final) and the content of V and the content of Ti satisfies the following formulas:

T_(start)=1100+a([V]+5[Ti]),

T_(final)=750+b([V]+5[Ti]),

wherein, 500≤a≤800, 300≤b≤500;

based on its total weight, the steel billet contains V of 0.03-0.08 wt %, Ti of 0.011-0.02 wt %, and [V]+5[Ti] of 0.12-0.14 wt %.

The invention also provides a hypereutectoid rail manufactured with the above method.

The hypereutectoid rail manufactured with the method of the invention has excellent comprehensive performance of obdurability (such as toughness and plasticity and contact fatigue resistance). In particular, it is able to obtain the hypereutectoid rail with the microstructure of pearlite and traces of secondary cementite.

Other features and advantages of the invention will be specifically described in the Detailed Description of the Preferred Embodiments later.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are provided for further understanding the invention and form a part of the specification. They are used for explaining the invention in combination with the following embodiments, and do not constitute any limitation to the invention. In the drawings:

FIG. 1 is the microstructure picture of the hypereutectoid rail obtained in the embodiment 1 of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is the detailed description for the embodiments of the invention. It should be understood that the embodiments described herein are only for explaining and illustrating the invention, and do not constitute any limitation to the invention.

The end-points of ranges and any values disclosed in the invention should not be limited to the precise ranges or values, which should be understood to contain values close to these ranges or values. For the range of values, one or multiple new ranges of values can be obtained through inter-combination among end-point values of ranges, among end-point values of ranges and individual point values as well as among individual point values, and these ranges of values shall be deemed to be specifically disclosed in the invention.

The invention provides a manufacturing method for hypereutectoid rail, wherein, the method comprises rolling a steel billet containing V and Ti, wherein, based on its total weight, the steel billet contains C of 0.85-0.94 wt %, and the relation between the start rolling temperature T_(start)/final rolling temperature T_(ris) and the content of V and the content of Ti satisfies the following formulas:

T_(start)=1100+a([V]+5[Ti]),

T_(final)=750+b([V]+5[Ti]),

wherein, 500≤a≤800, 300≤b≤500;

based on its total weight, the steel billet contains V of 0.03-0.08 wt %, Ti of 0.011-0.02 wt %, and [V]+5[Ti] of 0.12-0.14 wt %.

C is the most important and cheapest element to improve strength and hardness of pearlitic rail and to promote pearlitic transformation. When the content of C is <0.85%, the high strength and corresponding wear resistance required for heavy-haul railway cannot be obtained even though accelerated cooling is adopted after rolling; when the content of C is >0.94%, secondary cementite distributed along the grain boundaries will still be precipitated at the original austenite grain boundaries, deteriorating the impact toughness of rail and significantly degrading the contact fatigue resistance. Therefore, the content of C is limited within the range of 0.85%-0.94%.

V has low solubility in steel when under room temperature, and if V is present in austenite grain boundaries and other zones during hot rolling, it is precipitated through fine-grained V carbonitride or together with Ti in steel, suppressing the growth of austenite grains and thus refining grain and improving performance. When [V] is <0.03 wt %, the precipitation of V carbonitride is limited and the rail cannot be strengthened effectively; when [V] is >0.08 wt %, coarse carbonitride is prone to form, thus deteriorating the toughness and plasticity of the rail. Therefore, [V] is limited within the range of 0.03-0.08 wt %.

The main function of Ti in steel is to refine austenite grains during heating, rolling and cooling, and finally to improve the extensibility and rigidity of rail. It is one of the important elements added in the invention. When [Ti] is <0.011 wt %, the amount of carbides formed in the rail is extremely limited; when [Ti] is >0.02 wt %, on the one hand, as Ti is a strong carbonitride former, excessive TiC formed will make the hardness of the rail too high, and on the other hand, excessive TiC may lead to segregation enrichment and form coarse carbonitride, not only degrading the toughness and plasticity but also making the contact surface of the rail prone to crack under impact load and leading to fracture. Therefore, [Ti] is limited within the range of 0.011-0.02 wt %.

There are obvious differences between the affinity of V and Ti in steel with C, N and etc. as well as the quantity and form of carbides formed, but they have similar functions for forming carbonitride and refining austenite grains. Taking V as an example, in the steel with low content of N, the content of V dissolved in ferritic matrix through solid solution exceeds 50%; while in the steel with high content of N, the content of V dissolved in the steel through solid solution is over 20%, and the remaining 70% precipitated in the form of V carbonitride. In the invention, no significant improvement of performance occurs by adding either V or Ti. For example, if V of 0.09% is added without Ti, the strength of rail can still reach 1350 MPa, but the extensibility is generally lower than 10%; if Ti is added individually for microalloying, the strength of steel cannot reach the required 1350 MPa. Therefore, the steel billet of the invention contains V and Ti simultaneously and the content of [V]+5[Ti] is 0.12-0.14 wt %.

Preferably, the content of V is 0.045-0.055 wt %, the content of Ti is 0.014-0.02 wt %, and the content of [V]+5[Ti] is 0.12-0.13 wt %.

According to the invention, the steel billet with the above composition can be obtained through conventional methods in the art. For example, the steel billet of the invention can be obtained with the following method: smelting the molten steel with the above composition with a converter or electric furnace, conducting Al-free deoxidation, ladle refining and vacuum degassing treatment, and continuously casting to steel billet with a section of 250 mm×250 mm−450 mm×450 mm, then cooling the billet and heating it in a heating furnace with a heating temperature of 1200° C. above, insulating it for no more than 3 h to ensure uniform temperature of blanks of the billet section, then taking out the billet and removing oxide scale. The specific process will not be described herein.

According to the invention, during the process of rolling steel billet into rail, with different start rolling temperature, final rolling temperature and different [V] and [Ti], the size, distribution and morphology of precipitates vary significantly, thus contributing to obvious difference of the performance of rails obtained. The inventor of the invention found out that, when the relation between the start rolling temperature T_(start)/final rolling temperature T_(final) and [V]+5[Ti] satisfies the above formulas, it could be guaranteed that the fine and dispersedly distributed V carbonitride and Ti carbonitride in steel can be fully dissolved and precipitated, thus providing the hypereutectoid rail manufactured with excellent toughness, plasticity and contact fatigue resistance. In order to maximize the contribution of precipitates to the toughness and plasticity and fatigue resistance, preferably, in the above formulas, it must meet the conditions as 780≤a≤800 and 470≤b≤500.

According to the invention, based on its total weight, the steel billet may also contain Si of 0.4-0.9 wt %, Mn of 0.7-1.3 wt %, Cr of 0.2-0.6 wt %, P≤0.02 wt %, S≤0.008 wt % and N: 0.06-0.09 wt ‰.

According to the invention, as the solid-solution strengthening element of steel, Si is present in ferrite and austenite to improve strength of structure. The content of Si is preferably limited within the above range in the invention, which can help improve solid-solution effect, enhance toughness and plasticity of rail, optimize transverse performance of rail, and promote the safety use of rail.

According to the invention, Mn can form solid solution with Fe in steel billet, which can strengthen ferrite and austenite. Meanwhile, Mn is also a carbide former and can partially replace Fe atom after entering into cementite, improve hardness of carbide and finally improve hardness of the rail. The content of Mn is preferably limited within the above range in the invention, which can help ensure high strengthening effect, and meanwhile avoid affecting the toughness and plasticity due to high hardness of carbides in steel.

According to the invention, Cr can form a continuous solid solution with Fe in steel billet, and form a variety of carbides with C, and is one of the main strengthening elements of steel. Besides, Cr can produce even distribution of carbides in steel and improve the wear resistance of steel. The content of Cr is preferably limited within the above range in the invention, which can help improve the toughness and plasticity of rail.

According to the invention, the content of N is preferably limited within the above range, which can help improve the comprehensive performance of toughness and plasticity of rail under room temperature.

Further preferably, based on its total weight, the steel billet contains Si of 0.55-0.65 wt %, Mn of 1.25-1.3 wt %, Cr of 0.4-0.55 wt %, P≤0.014 wt %, S≤0.005 wt % and N of 0.06-0.07 wt ‰.

According to one preferred embodiment of the invention, the method may also comprise the following steps: carrying out rapid cooling for railhead when the surface temperature of the railhead T_(surface) drops to 20-50° C. below the final rolling temperature T_(final) after the rolling; ending the rapid cooling and continuing air-cooling the rail to room temperature when the surface temperature of the railhead T_(surface) drops to 450-550° C. due to the rapid cooling process.

According to the invention, by limiting the surface temperature of the railhead T_(surface) at the time when starting rapid cooling and ending rapid cooling within the above range, it can prevent the secondary cementite likely forms along the grain boundaries, which can help improve the toughness and plasticity of rail.

According to the invention, there is no particular limit to the cooling rate during the rapid cooling and a conventional cooling rate in the art may be applied. However, in order to reduce precipitation of secondary cementite and to improve the strength and wear resistance of rail, preferably, the cooling rate for the rapid cooling is 2-5° C./s, more preferably 4.5-4.9° C./s.

According to the invention, there is no particular limit to the conducting method of the rapid cooling and a conventional method in the art can be applied. For example, the method for conducting the rapid cooling can be realized by applying a cooling medium to the top surface of railhead and two sides of the rail.

According to the invention, there is no particular limit to the cooling medium and any conventional cooling medium in the art can be applied, as long as the purpose for the cooling in the invention can be achieved. Preferably, the cooling medium is compressed air and/or water-air spray mixture.

The invention also provides a hypereutectoid rail manufactured with the above method. It should be understood that the hypereutectoid rail provided in the invention has the same composition with the above steel billet. The hypereutectoid rail in the invention has excellent toughness and plasticity and contact fatigue resistance.

The invention will be described in detail through the embodiments. In the following embodiments, “room temperature” refers to “25° C.”.

The composition of the steel billet in the following embodiments and references is shown in table 1, wherein, in addition to the elements in table 1, the rest is Fe and inevitable impurities:

TABLE 1 Serial Chemical composition (wt %) (wt %) number C Si Mn P S V Ti [V] + 5[Ti] Cr N 1# 0.90 0.48 0.92 0.014 0.008 0.04 0.020 0.14 0.2 0.078 2# 0.85 0.90 1.00 0.012 0.006 0.07 0.013 0.135 0.6 0.09 3# 0.92 0.88 1.21 0.011 0.006 0.06 0.011 0.115 0.3 0.088 4# 0.88 0.72 0.88 0.011 0.006 0.08 0.010 0.13 0.25 0.082 5# 0.86 0.57 1.30 0.013 0.004 0.05 0.014 0.12 0.4 0.068 6# 0.94 0.61 0.70 0.010 0.007 0.03 0.020 0.13 0.55 0.06

Embodiments 1-6

The 1#-6# steel billets in the table 1 are rolled into 60 kg/m rails with the rolling and rapid cooling processes listed after the corresponding serial numbers in table 2, and are rapidly cooled with compressed air. The rails going through the above treatment are then air-cooled to room temperature to get hypereutectoid rails A1-A6.

TABLE 2 Start rolling Final rolling Temperature to Cooling rate Temperature to Serial temperature temperature start rapid during rapid stop rapid number T_(start) (° C.) T_(final) (° C.) cooling cooling cooling 1# 1170 (a = 500) 792 (b = 300) 772° C. 3.2° C./s 492° C. 2# 1184 (a = 620) 801 (b = 380) 771° C. 5.0° C./s 475° C. 3# 1190 (a = 780) 798 (b = 420) 763° C. 3.7° C./s 450° C. 4# 1194 (a = 720) 809 (b = 450) 759° C. 4.1° C./s 525° C. 5# 1196 (a = 800) 810 (b = 500) 770° C. 4.8° C./s 511° C. 6# 1175 (a = 580) 811 (b = 470) 761° C. 2.0° C./s 500° C.

References 1-6

The 1#-6# steel billets in the table 1 are rolled into 60 kg/m rails with the rolling and rapid cooling processes listed after the corresponding serial numbers in table 3, and are rapidly cooled with compressed air. The rails going through the above treatment are then air-cooled to room temperature to get hypereutectoid rails D1-D6.

TABLE 3 Start rolling Final rolling Temperature to Cooling rate Temperature temperature temperature start rapid during rapid to stop rapid Serial number T_(start) (° C.) T_(final) (° C.) cooling cooling cooling 1# 1250 888 806° C. 3.2° C./s 492° C. 2# 1235 874 799° C. 5.0° C./s 475° C. 3# 1218 901 782° C. 3.7° C./s 450° C. 4# 1206 892 820° C. 4.1° C./s 525° C. 5# 1240 893 815° C. 4.8° C./s 511° C. 6# 1224 909 802° C. 2.0° C./s 500° C.

Test Example 1

Performance test is conducted for the hypereutectoid rails A1-A6 and D1-D6 manufactured respectively in embodiments 1-6 and references 1-6 with the following method to specifically:

Determine the tensile property of the hypereutectoid rails according to GB/T228.1-2010, Metallic Materials—Tensile Testing—Part 1: Method of test at room temperature, and get R_(m) (tensile strength) and A % (extensibility), with results shown in table 4;

Determine the impact property (U-type) of railhead under room temperature of the hypereutectoid rails with conventional methods in the art, with results shown in table 4;

Determine the microstructures of the hypereutectoid rails by using a MeF3 optical microscope according to GB/T 13298-1991, Metal—Inspection Method of Microstructure, with results of the determined microstructures shown in table 4. The microstructure of A1 is shown in FIG. 1.

TABLE 4 Impact property (U-type) of railhead Serial Tensile properties under room number R_(m) (MPa) A (%) temperature (J) Microstructure A1 1387 11.5 21 P + Fe₃C_(II) (traces) A2 1392 12.0 19 P + Fe₃C_(II) (traces) A3 1405 12.5 19 P + Fe₃C_(II) (traces) A4 1410 12.0 18 P + Fe₃C_(II) (traces) A5 1368 11.5 22 P + Fe₃C_(II) (traces) A6 1398 12.5 20 P + Fe₃C_(II) (traces) D1 1405 9.5 8.5 P + Fe₃C_(II) (small amount) D2 1382 9.0 9.0 P + Fe₃C_(II) (small amount) D3 1410 8.5 9.5 P + Fe₃CII (small amount) D4 1408 8.5 9.0 P + Fe₃C_(II) (small amount) D5 1372 8.0 8.0 P + Fe₃C_(II) (small amount) D6 1387 9.0 7.5 P + Fe₃C_(II) (small amount) Note: P + Fe₃C_(II) (traces) refers to pearlite and traces of secondary cementite and P + Fe₃C_(II) (small amount) refers to pearlite and small amount of secondary cementite. “Traces” and “small amount” have relative relations and “traces” of quantity is less than “small amount” of quantity, mainly to indicate that the secondary cementite in the microstructure of the hypereutectoid rails in the embodiments is less than that in the references.

It can be concluded from the comparison results between embodiments 1-6 and references 1-6 that, the microstructure test results indicate that all the structures of rails are pearlite and secondary cementite (Fe₃C_(II)), but the difference lies in that less secondary cementite is precipitated along grain boundaries by applying the composition and the corresponding processes of the invention, which is more favorable for improving the toughness and plasticity of rail and the service safety of rail. Based on the performance indexes, under the condition that the tensile strength of the rails in the embodiments is close to that in the references, the extensibility and the impact property (U-type) of railhead under room temperature of the hypereutectoid rails in the invention are greatly improved, which is more favorable for improving the contact fatigue resistance of rail.

The above is a detailed description of the preferred embodiments of the invention, However, the invention shall not be restricted to the specific details of the above embodiments. Within the scope of the technical concept of the invention, multiple simple variations can be conducted for the technical scheme of the invention, which shall all fall within the protection scope of the invention.

Moreover, it should be specified that, the specific technical characteristics described in the specific embodiments above can be combined in any proper manner without contradiction. To avoid unnecessary repetition, the various possible combinations are not described in the invention.

In addition, any embodiments of the invention can also be combined, and such combination shall also be deemed as the content disclosed by the invention as long as it does not depart from the concept of the invention. 

1. A manufacturing method for hypereutectoid rail, said method comprising rolling a steel billet containing V and Ti, wherein, based on a total weight of the steel billet, the steel billet contains C of 0.85-0.94 wt %, and a relationship between a start rolling temperature T_(start)/a final rolling temperature T_(final) and a content of V and a content of Ti satisfies the following formulas: T_(start)=1100+a([V]+5[Ti]), T_(final)=750+b([V]+5[Ti]), wherein 500≤a≤800, 300≤b≤500; and based on the total weight of the steel billet, the steel billet contains [V] of 0.03-0.08 wt %, [Ti] of 0.011-0.02 wt %, and [V]+5[Ti] of 0.12-0.14 wt %.
 2. The method according to claim 1, wherein, based on the total weight of the steel billet, the steel billet contains [V] of 0.045-0.055 wt %, [Ti] of 0.014-0.02 wt %, and [V]+5[Ti] of 0.12-0.13 wt %.
 3. The method according to claim 1, wherein, based on the total weight of the steel billet, the steel billet also contains Si of 0.4-0.9 wt %, Mn of 0.7-1.3 wt %, Cr of 0.2-0.6 wt %, P≤0.02 wt %, S≤0.008 wt % and N of 0.06-0.09 wt %.
 4. The method according to claim 3, wherein, based on the total weight of the steel billet, the steel billet contains Si of 0.55-0.65 wt %, Mn of 1.25-1.3 wt %, Cr of 0.4-0.55 wt %, P≤0.014 wt %, S≤0.005 wt % and N of 0.06-0.07 wt %.
 5. The method according to claim 1, wherein the method also comprises the following steps: carrying out rapid cooling for a railhead when a surface temperature of the railhead T_(surface) drops to 20-50° C. below the final rolling temperature T_(final) after the rolling; ending the rapid cooling and continuing air-cooling the rail to room temperature when the surface temperature of the railhead T_(surface) drops to 450-550° C. due to the rapid cooling process.
 6. The method according to claim 5, wherein a the cooling rate for the rapid cooling is 2-5° C./s.
 7. The method according to claim 6, wherein the cooling rate for the rapid cooling is 4.5-4.9° C./s.
 8. The method according to claim 5, wherein the method for the rapid cooling is to apply a cooling medium to a top surface of the railhead and two sides of the rail.
 9. The method according to claim 8, wherein the cooling medium is compressed air and/or a water-air spray mixture.
 10. A hypereutectoid rail manufactured with the method according to claim
 1. 